1. Field of the Invention
A method for deriving and using static corrections for the effects of dispersion due to anelasticity as applied to seismic signal data processing.
2. Discussion of Related Art
As is well known in the art of seismic exploration of the earth and shown in FIG. 1, an acoustic source, at or near the surface of the earth, repeatedly emits a wavefield from each of a plurality of seismic source locations, S.sub.i, to insonify the subsurface earth layers, L.sub.i. The wavefield is reflected from the respective earth layers. Returning to the surface, the reflected wavefields are detected by a plurality of suitable receivers, R.sub.j, such as geophones or hydrophones which are deployed at preferred spatial intervals over an area of survey. The offset, O.sub.k, that is, the separation between a source S.sub.i and some receiver R.sub.j, typically ranges from a few hundred meters to 3000 or more meters. The separation between the individual receivers is termed the group interval which typically is about 100 meters. The separation between source locations is may be a multiple of the group interval. Often the multiple is unity.
The mechanical acoustic waves resulting from the reflected wavefields are detected by the receivers and converted to analog electrical signals which may be discretized and transmitted to a multichannel, central signal-recording device. Usually each receiver or receiver group is interconnected with a dedicated amplifier channel in the recording device via a signal transmission means which may be electrical, optical or ethereal. Each receiver may transmit its data through an individual transmission channel or a number of receivers may be time- or frequency-multiplexed into a single shared transmission means. As many as a thousand separate channels may be used to provide a single seismic record. Depending upon the total number of channels to be serviced, the effective listening time and the preferred data resolution, the analog data signals may be discretized at 0.5 to 4.0 millisecond (ms) intervals. The recorded data signals are formatted as a plurality of time-scale traces, one trace per channel, to provide recordings of two-way reflection travel time as a function of receiver offset.
Many of the wavefield trajectories between the various source locations and the respective receivers are redundant thereby to provide manifold common coverage groupings or gathers. Selected gathers having a desired multiplicity may include, by way of example but not by way of limitation, common depth point, common mid point, common receiver, common source, and common offset gathers.
The recorded seismic data signals are processed by suitable well-known means, usually by a programmed computer which may be coupled to a graphics processor, to convert the seismic signals into a different state such as a graphic 2- 3-dimensional scale model of the subsurface of the earth in a region of interest. Signal data processing may include (although not necessarily in the order listed below) electrical and/or digital filtering, trace stacking (summation) of data from a selected common coverage grouping, deconvolution, application of angularity and dip-related corrections (NMO and DMO), migration, correction for spherical spreading, time-depth conversion, corrections for instrumental artifacts and truncation transients, seismic noise, induced atmospheric transients and surface-consistent static correction. Generally in exploratory seismic studies, the media traversed by the wavefield trajectories are assumed to be non-dispersive.
The term "static correction" refers to erratic travel-time delays due to abrupt variations in properties of the near-surface low-velocity layer through which the wavefield trajectory travels between the source(s) and the receivers. The travel-time variations may be due to changes in layer thickness or changes in layer velocity or both. No matter how sophisticated may be the data-processing routine, in the absence of accurately-applied static corrections, the seismic data presentation will border on the useless.
Static corrections to be applied to common coverage wavefield trajectories are assumed to be surface-consistent. That is, static corrections are separately attributable to the locations of a specific source or a specific receiver.
In days of yore, static corrections were measured manually from near surface refracted travel times such as taught by H. Salvatori et al. in U.S. Pat. No. 2,087,120, issued Jul. 13, 1937, for A Method for Computing Weathering Corrections in Seismic Surveying and assigned to a predecessor firm of the assignee of this invention. Alternatively static corrections were sometimes determined by inspection of key reflections from the seismic data recordings. As the seismic exploration production rate increased by several orders of magnitude, manual empirical measurements gave way to the computer where the relative static delays are determined such as by intertrace cross-correlation. One well-known commercial process is the MISER.RTM. (trademark of Western Geophysical Co.) residual static correction program and described in a paper entitled Residual Static correction Analysis As A General Linear Inverse Problem, published in Geophysics, v. 41, n. 5 1976, by Ralph A. Wiggins et al. In that method for determining surface-consistent static corrections, seismic velocities in the regosol were assumed to be non-dispersive, that is, independent of frequency.
In attempting to account for otherwise unexplained data misties following meticulous application of known processing methods, we have discovered that the regosol may be anelastic in places. Anelasticity results in velocity dispersion wherein acoustic propagation velocity varies with respect to frequency which, in turn causes waveform distortion. There is a need for a method for determining surface-consistent static correction in dispersive-media.